Weathering of fuel oil spill on the east Mediterranean coast,
Ashdod, Israel
Shai Ezra
a,*, Shimon Feinstein
a, Ithamar Pelly
a, Dan Bauman
b,
Irena Miloslavsky
caDepartment of Geological and Environmental Sciences, Ben-Gurion University of the Negev, Beer Sheva, 84105, Israel bDepartment of Geography and Environmental Development, Ben-Gurion University of the Negev, Beer Sheva, 84105, Israel
cDepartment of Organic Chemistry and Casali Institute, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel
Abstract
Residual fuel oil spilled into the sea from the Eshkol power station on 8 February, 1998 contaminated about 9 km of the foreshore north of the Ashdod harbour. A study of the aliphatic, polycyclic alkane and polyaromatic hydrocarbon (PAH) composition of the spilled oil shows rapid weathering in the early stages followed by gradual slowdown after about three months. Weathering of isoprenoids and PAH compounds and variation in Pr/Ph ratio appear to occur almost contemporaneously with that ofn-alkanes, at a relatively moderate level of degradation, when much of the >C20 n-alkane envelope is still well preserved. Depletion of various compounds in accordance with molecular size
rather than molecular structure appears to imply that physical weathering processes, i.e. evaporation and perhaps ¯ushing due to wave energy, might have played an important role in the degradation of the spilled residual fuel oil in this study case.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Fuel oil spill; Pollution; Weathering; Hydrocarbons; East Mediterranean foreshore
1. Introduction
Extensive transportation of petroleum by ocean going tankers, oil exploration and production activities, and the frequent location of oil consuming industries in coastal areas make marine and coastal environments particularly vulnerable to pollution by petroleum and its breakdown products. Oil spilled into the environment is subjected to a variety of weathering processes, including evaporation, dissolution, dispersion, photochemical oxidation, ¯ushing due to wave energy, emulsi®cation, microbial biodegradation and adsorption to suspended matter and deposition on to the sea¯oor (Bohem et al., 1982; Wang et al., 1997b; Garret et al., 1998). Weath-ering processes involved, together with the chemical nature of the spilled oil, determine the fate and rate of degradation. The weathering processes under natural conditions are complicated and depend on a variety of
factors. Therefore, studies of weathering under natural conditions are essential for the understanding of these processes in the natural environment. Each weathering process may have a dierent eect on the oil compo-nents (e.g. Connan et al., 1980; Boehm et al., 1982; Volkman et al., 1984; Rowland et al., 1986; Wang and Fingas, 1995a,b; Fisher et al., 1996; Budzinski et al., 1998; Sauer et al., 1998; Garrett et al., 1998). Thus, variation over time in the composition of dierent oil constituents can be used to monitor the extent of weathering. A detailed understanding of weathering processes that oil is subjected to is required in order to reduce environmental damage and develop eective protection strategies.
On 8 February, 1998 residual fuel oil from the Eshkol power station, north of Ashdod harbour, spilled as a result of a technical failure. The fuel spilled into the sea through the cooling water outlet system and formed an elongated ¯oating lens in the nearshore that was trans-ported by the waves. It also landed on the beach and polluted a 9 km stretch of the coastline. The coastline polluted by the spill is generally characterized by sandy
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beaches with scattered beach rock boulders, partially exposed due to sand mining in the 1950s. The spill pro-vided an excellent opportunity to monitor, with relatively high resolution, the variation in the chemical composi-tion of the spilled residual oil over time and characterize the weathering processes in the Israeli coastline of the east Mediterranean under natural conditions. Systema-tic sampling at the polluted shore started about 48 h after the spillage began, at a time when the beaching of tar was still occurring. Moreover, knowledge of the source fuel provided a reference composition and the possibility to extend the control on the weathering pro-cesses back in time to their initiation (`zero time').
2. Experimental
2.1. Samples
Systematic sampling of the polluted shore stretch commenced 48 h after the spill began and continued for more than a year. The residual oil pollution (tar pat-ches) was sampled at three stations (Fig. 1), each com-prising several sampling points of sand and beach rocks. Sand sampling points lasted for only about one month, before they were washed away. Contamination patches
on rocks were persistent throughout the study period and were sampled continually for 14 months. Sampling was carried out at 2 weekly intervals during the ®rst 4 months and subsequently once a month, on the basis of the initial results. A sample of the initial residual fuel oil was obtained from the Eshkol power station fuel container. Samples were stored in sealed vials and kept at 5C.
2.2. Extraction and liquid chromatography
The samples were solvent extracted with toluene. Asphaltenes were precipitated from the extracted frac-tion with n-hexane (50:1 hexane:toluene). The deas-phalted extracts were fractionated using column chromatography (30 cm length and 1 cm width column ®lled half with activated alumina and half with activated silica gel). Approximately 100 mg of deasphaltened fraction was adsorbed on to the top of the column. The aliphatic fraction was recovered by eluting with hexane (300 ml), the aromatic fraction with toluene (300 ml) and the resins with methanol (200 ml).
2.3. Gas chromatography (GC)
Aliphatic fractions were analyzed on a Hewlett Pack-ard (HP) 5890 FID gas chromatograph with a HP-5 fused silica column (30 m, 0.32 mm i.d., 0.25mm ®lm). The injector and the detector were held at 300C and the
column temperature program was 50C (2 min),
fol-lowed by heating to 300C at 4C/min (isothermal hold
for 20 min). The carrier gas used was He.
2.4. Gas chromatography±mass spectrometry (GC±MS)
Aliphatic and aromatic fractions of selected samples were analyzed by GC±MS. Scanning of polycyclic alkanes was performed using a HP 57904A GC directly connected to a ZAB- II F (VG analytical) double-focus-ing MS. The GC was equipped with a DB-5 fused silica column (30 m, 0.32 mm i.d., 0.25mm ®lm). The injector and the detector were held at 300C. The MS operated in
selected ion resolution (SIR) mode. The ions monitored werem/z 191.1800 for hopanes andm/z 217.1956 and 218.2035 for steranes. To obtain spectral data and iden-ti®cation of the PAH compounds a HP 5890 GC directly connected to an HP 5971A MS was used. The GC was equipped with a HP-1 fused silica column (30 m, 0.20 mm i.d., 0.33 mm ®lm). The injector and the detector were held at 280 and 250C, respectively. The MS operated in
a single ion monitoring (SIM) mode. The ions monitored werem/z128, 142 and 156 for naphthalene and alkyl-naphthalenes; m/z 178, 192 and 206 for phenanthrene and alkylphenanthrenes; m/z 184, 198 and 212 for dibenzothiophene and alkyldibenzothiophenes; andm/z
166, 180 and 194 for ¯uorene and alkyl¯uorenes. The column temperature program for both GC±MS
ments was at 50C (2 min), followed by heating to
300C at a 4C/min rate (isothermal hold 20 min).
3. Results
3.1. Aliphatics
Fig. 2a±c shows C15+ gas chromatograms obtained
for then-and isoalkanes in the residual fuel oil sampled from the power station container and two tar samples (station AC) obtained at 3 and 30 weeks after the spill. The n-alkane envelope in the residual fuel oil (Mazut) from the power station container is characterized byn -alkanes ranging from C13 to C35 maximizing at n-C22
(Fig. 2a). Subsequently, there is an absence of aliphatic compounds <n-C15after 3 weeks (Fig. 2b) and aliphatic
compounds <n-C17 30 weeks after the spill (Fig. 2c).
Maxima are also changed, from n-C22 in the ®rst few
weeks (Fig. 2a and b) ton-C24after 30 weeks (Fig. 2c).
Likewise, Fig. 2a±c shows an increase in the unresolved complex mixture (UCM) in the gas chromatograms.
3.2. Polyaromatic hydrocarbons (PAHs)
Fig. 3 shows summed mass chromatograms obtained for target PAHs in the residual fuel oil (Fig. 3a) and in
two tar samples taken 6 and 50 weeks after the spill (Fig. 3b and c, respectively). The dominant PAHs in the residual fuel oil are naphthalenes and phenanthrenes. Dibenzothiophenes (DBTs) are considerably less abun-dant. Likewise, the relative abundance of ¯uorenes is also low. The relative abundance of naphthalenes declines rapidly with time in tar samples and at most samples was below the detection limit after 2 weeks. The order of decrease of other PAH compounds was in accord with their molecular weight, i.e. the number of rings and/or size of the alkylated homologues. On the other hand, the distribution of dierent isomers appears to remain steady throughout the study period. Fig. 4 shows the constancy of the isomer ratio through time for the methyldibenzothiophenes (MDBTs).
3.3. Polycyclic alkanes
Unlike the aliphatic and PAH distributions, hopane and sterane distributions obtained for the residual fuel oil and all tar samples analyzed were similar. Fig. 5 shows the hopane distribution of the residual fuel oil (Fig. 5a) and of tar samples taken 2 (Fig. 5b) and 20 weeks (Fig. 5c) after the spill. The distributions in all three samples are similar and are characterized by a C30
dominance over C29, an abundance of gammacerane,
and C31 homohopane epimers ratio [22S/(22R+22S)]
ranging from 0.56 to 0.59 for all samples studied. The
bb-steranes distributions are characterized by a pre-dominance of C29over C28and C27and by similar
con-centrations of C28and C27steranes.
4. Discussion
This study attempts to characterize the natural weathering processes of residual fuel oil pollution in a foreshore environment by detailed monitoring of chan-ges in its chemical composition with time. We assume
Fig. 3. Mass chromatograms of alkylated PAHs. Chromato-grams are composed of SIMm/z128, 142, 156 (naphthalene and alkyl homologues);m/z178, 192, 206 (phenanthrene and alkyl homologues);m/z 184, 198, 212 (dibenzothiophene and alkyl homologues); andm/z166, 180, 194 (¯uorene and alkyl homo-logues). (a) source fuel oil (Mazut);(b) six weeks after the spill, and (c) a year after the spill. Note the depletion of methyl-naphthalene, phenanthrene and methylphenanthrene with time.
Fig. 4. Methyldibenzothiophene isomers. MDBT=methyldi-benzothiophene.
that variations in the chemical characteristics of the residual oil pollution (tar) samples originating from the same source represent the dierential eect of weath-ering processes.
Sampling at the three stations was carried out repeatedly on the same tar patches, hence a consistent source for the samples within each set can be con®dently assumed. Nevertheless, this does not necessarily mean that the dierent patches sampled at the three sampling stations along the coast (Fig. 1) are all related to the same source, nor that the residual fuel oil spilled on 8 February, 1998 is the source of them all. It is important to determine whether there is a common source in order to establish a reference fuel composition (before altera-tion commenced) and to enable the correlaaltera-tion of results obtained for samples from dierent patches.
The polluted coastal stretch north of the city of Ash-dod is a populated region and is easily accessible. Therefore, the contamination process was observed a short time after spillage at the Eshkol power station began. The 9 km long polluted stretch was formed on the foreshore within a short time interval at a uniform altitude (which records a uniquely high sea level at the time the oil landed at the seaside) all along the polluted interval.
Spilled oil is exposed to modi®cation of its chemical characteristics through various processes of degrada-tion. Hence, correlation for a genetic relation is restric-ted to parameters suciently resistant to degradation, thus retaining a chemical `®ngerprint' inherited from the source oil. Hopanes and steranes are among the most useful `biomarkers' recording characteristics of their genetic history and among the most resistant to sec-ondary modi®cations of the petroleum hydrocarbon compounds (e.g. Seifert and Moldowan, 1979; Connan et al., 1980; Hostettler and Kvenvolden, 1994; Wang et al., 1994; Kvenvolden et al., 1995; Wang et al., 1995; 1997b). Hopane and sterane distributions of the tar samples from the three stations at dierent sampling times and from the tank fuel oil at the Eshkol power station correlate well (Fig. 5).
A good correlation is also shown for other molecular characteristics with a lower resistance to weathering (e.g. MDBT isomers ratio Ð Fig. 4), although, at later stages correlation of these compounds becomes more dicult, in accordance with their dierential suscept-ibility to weathering. However, such compounds in samples from relatively early stages can still be useful for correlation in spite of the fact that later on they might be susceptible to weathering. In this regard, it is pertinent to note that diagrams describing variations in composition with time converge back, at the time of the initiation of weathering, to a composition similar to that characterizing the container fuel oil (e.g. Figs. 6 and 7). The correlation of chemical characteristics together with ®eld observations strongly corroborates the supposition
of a common origin for the samples from the three sampling stations and of their origin from the 8 Feb-ruary, 1998 spill at the Eshkol power station.
Figs. 2 and 3 show the modi®cation in the `envelope' of the aliphatic compounds and in the PAH distribution with time; in both cases there is a gradual removal of the lower molecular weight compounds. The major change in the aliphatic hydrocarbons in the ®rst few weeks is characterized by a decrease in the relative abundance of both normal and iso-alkanes, mainly in the <n-C17
range (Fig. 2b). These changes are already apparent 48 h after the spill. Subsequently, the weathering gradually evolves into a reduction ofn-and iso-alkanes up to ca. C22(Fig. 2c).
Figs. 6±8 describe variations in the composition with time ofn-alkanes and isoprenoids. Ratios in these ®g-ures are calculated based on peak height in the C15+
chromatograms with no calibration to actual con-centrations and, therefore, they only represent an approximation of the relative concentration of each component. The rate ofn-alkane weathering is rapid in the ®rst few weeks and subsequently slows down until after about three to four months it becomes nearly con-sistent (Fig. 6). At this stage, the normal and iso-alkanes in the <C17range are almost completely removed and
there is a considerable depletion ofn-C17to ca.n-C20.
Fig. 2 further demonstrates that in this study, iso-prenoid compounds are depleted by the weathering in a similar way to that of the n-alkanes in the roughly equivalent molecular weight range, but with a small time lag. As an example,n-C17is depleted slightly earlier
than pristane and perhaps at a somewhat faster rate (Fig. 7). Nevertheless, pristane is depleted earlier and at a signi®cantly faster rate than n-alkanes at the C20+
range (Fig. 7). Perhaps an even more unique observa-tion in the pattern of isoprenoid weathering in this study is the change with time in the ratio of pristane to phy-tane. Fig. 8 demonstrates a clear systematic and rela-tively rapid change in the Pr/Ph ratio within about 3 months after the spill, from1.1 to below 0.5, a level at
which it appears to stabilize. The rate of change in Fig. 8 re¯ects the dierence in pristane and phytane weath-ering rate under the conditions of this study. Thus, in this study, weathering aects both the amount of each isoprenoid and the Pr/Ph ratios and it occurs at a rela-tively rapid rate at a moderate level of weathering.
Likewise, phenanthrene is hardly noted in tar sampled after 6 weeks (Fig. 3b), and methylphenanthrenes after 50 weeks from the spill (Fig. 3c). On the other hand, unlike the preferential degradation of alkyl homologues, there appears to be no preference for degradation between dierent isomers of the compounds studied (Fig. 4).
Most of the weathering observed in the aliphatic and PAH compounds occurs within a relatively short time and after several months the rate slows down to nearly zero (Figs. 6±8). After about a year from the spill the aliphatic fraction in tar samples still retains a prominent
n-C22+ envelope and the PAH distribution comprises
C2-phenanthrene and C2-DBT (Figs. 2 and 3).
Natural environments comprise complex and dyna-mically varying conditions and processes which are often dicult to monitor adequately. The source of the tar patches is a relatively heavy fuel oil. Tar samples
studied herein are from the foreshore zone. Among the major environmental parameters under which the weathering evolved are: atmospheric pressure, moderate temperatures with daily and seasonal variations ranging between 5 and 40C and an excess of oxygen. The fuel
oil was transported through sea water with a residence time of up to several days. Since beaching, it was exposed to alternating dry and wet conditions, frequent ¯ushing of variable energy, and sometimes to temporal cover by sediments. Thus, almost any weathering mechanism (e.g. biodegradation, evaporation, dissolu-tion, photo-chemical decomposidissolu-tion, emulsi®cation and ¯ushing due to wave energy) could feasibly have occur-red.
The pattern and rates of weathering obtained for the various sampling stations (Fig. 1) are similar (Figs. 6±8). Hence, we assume that to the limit of the resolution of the study herein, the 9 km polluted shoreline represents
Fig. 7. Variation inn-C17to Pr and Pr ton-C22ÿ30ratios through time.
Fig. 6. Variation in n-C15ÿ30ton-C22ÿ30ratio through time at the three sampling sites:*Mazut;*northern sampling station;
a relatively homogenous habitat with similar weathering conditions. The decrease with time in the ratio of rela-tively low to high molecular weight n-alkanes (Fig. 6) can be, also, an indication of microbial degradation (Bohem et al., 1982; Sauer et al., 1998). Nevertheless, most studies stress the relative resistance of isoprenoids to microbial degradation and consequently an increase in their relative concentration, due to the dierential depletion of n-alkanes (Bohem et al., 1982; Connan, 1984; Pritchard and Costa, 1991; Hostettler and Kven-volden, 1994; Wang et al., 1997b; Sauer et al., 1998). Apparently, characteristics observed in the present study do not conform to this pattern. Although the ratio of Pr/n-C17 and Ph/n-C18, which indicate increasing
biodegradation, rises (Fig. 7), generally, isoprenoids are depleted at a rate comparable to their molecular weight equivalent n-alkanes (Figs. 2 and 7). Likewise, within this time range PAHs also undergo eective weathering (Fig. 3a and b). PAHs are generally believed to be more resistant to biodegradation than aliphatics. Generally, in this group the relative susceptibility to biodegrada-tion decreases with the number of aromatic rings and the number of alkyl substitution (Volkman et al., 1984; Rowland et al., 1986; Fisher et al., 1996; Budzinski et al., 1998). Nevertheless, the resistance to biodegradation of each individual alkyl PAH is dierent (Rowland et al., 1986; Wang and Fingas, 1995b; Fisher et al., 1996; Budzinski et al., 1998). For example, Wang and Fingas (1995b) show, using simulation experiments, that bio-degradation has a dierent eect on the isomer dis-tribution of MDBT mainly by way of a relative depletion of 2/3-MDBT. In our study, isomeric ratios within a particular PAH molecular group, including MDBT, appear to remain unaected (Fig. 4). Thus, although biodegradation might have played a partial role in the weathering process, it is evidently not the sole
process, and probably not the dominant one. A rela-tively restricted role of microbial degradation is in agreement with other studies. As an example, Boehm et al. (1982) suggest that bacterial degradation did not play a major role in the case of 1979 Ixtoc blowout in the Bay of Campeche. They attribute this to possible nutri-ent de®ciency. Wang et al. (1997a) show that for resi-dual oils, as in the case of the present study, biodegradation results in relatively little weight loss.
Dissolution and photochemical oxidation are among the weathering mechanisms under which PAHs are readily degraded, whereas saturated compounds are relatively resistant (Boehm et al., 1982; Connan, 1984; Payne and Phillips, 1985; Garret et al., 1998). Never-theless, Garret et al. (1998) show that the sensitivity of PAHs to photooxidation increases with molecular size and alkyl substitution, a trend opposite to that recorded here (Fig. 3). Solubility of PAHs is greater than that of saturated compounds and among the PAHs decreases with molecular weight (Eganhouse and Calder, 1976; Boehm et al., 1982; Connan, 1984). Thus, the trend of PAH depletion in this study (Fig. 3) is consistent with that anticipated for dissolution, but not that for the contemporaneous depletion of relatively heavy
n-alkanes and isoprenoids (Figs. 2, 6 and 7) (Sutton and Calder, 1974).
The dominant feature in the pattern of changes with time in the various groups appears to be the molecular weight rather than molecular structure. This is illu-strated by the fact that the relative abundances of iso-prenoids and PAHs are extensively reduced while normal alkanes in the C22+ range appear largely
unaf-fected (Fig. 2). This characteristic might also be re¯ected in the remarkable decrease in Pr/Ph ratio (Fig. 8) and the dierential depletion of PAHs in accordance with the number of rings and the size of the alkyl groups in
the molecule (Fig. 3). This pattern of compositional changes, i.e. contemporaneous depletion of aliphatic and PAH compounds in accordance with their mole-cular weight, is compatible with that produced by eva-poration (Boehm et al., 1982; Wang and Fingas, 1995a). Both ®eld observations (Boehm et al., 1982) and laboratory experiments (Wang and Fingas, 1995a) show eective depletion of both aliphatic and aromatic com-ponents with a rate decreasing with molecular weight. Various studies also show that evaporation results in depletion of pristane and phytane at a rate similar to the
n-C17 and n-C18 alkanes (Wang and Fingas, 1995a;
Sauer et al., 1998).
The possibility implied by the pattern of the chemical changes that evaporation is a major weathering mechanism in the present study is concordant with other case studies (e.g. Bohem et al., 1982; Strain, 1986; Sauer, 1998). In the ®eld studies of Boehm et al. (1982) and Wang and Fingas (1995a), evaporation of the PAHs appears to aect mainly the naphthalenes, ¯uorenes and DBTs, with only a small eect on the phenanthrenes and on the aliphatics up to C17and with no distinctive
eect beyond C20ÿ22range. In the present study,
weath-ering appears to aect somewhat heavier molecules such asn-C21, phenanthrene and methylphenanthrene (Figs.
2 and 3). The implication of this dierence is somewhat puzzling. Wang and Fingas (1995a) performed their experiments at ca. 60C. Temperatures of tar patches in
the studied stations were not measured, but should be lower than that, particularly during the ®rst few months (winter season). A similar and even more extensive deple-tion of three-ring PAH compounds and perhaps even some chrysenes is reported at the Saudi Arabian coast (Sauer et al., 1998) where temperatures are presumably somewhat higher than in the study area. In any case, eective removal of molecules like phenanthrene and aliphatics up to C20with a vapor pressure in the order
of 10ÿ7mm and less, at such a low temperature range,
appears enigmatic. An alternative mechanism that might contribute to the weathering, although at present entirely speculative, might be a dierential washing out of fractions with greater ¯uidity due to wave energy, again, mainly a function of molecular weight.
5. Conclusions
Systematic monitoring of changes with time in the chemical composition of a residual oil spill on the East Mediterranean foreshore north of the Ashdod Harbour reveals:
(a) A relatively rapid weathering during the ®rst few months. The weathering gradually slows down and becomes very slow after about three months.
(b) Weathering of the aliphatic fraction is involved, with a substantial change in the isoprenoid content and
Pr/Ph ratio at a relatively moderate level of degradation, when much of the >C20n-alkanes envelope is still well
preserved.
(c) Depletion of PAH compounds occurs approxi-mately simultaneously with the normal and iso-alkanes. Weathering of the PAHs evolves in accordance with their molecular size, number of rings and number of alkyl constituents. There appears to be no preferential weathering of dierent isomers of a molecule.
(d) The approximately contemporaneous depletion of normal alkanes, isoprenoids and PAH compounds, in accordance with molecular weight rather than molecular structure, appears to imply that physical weathering processes, evaporation and perhaps ¯ushing due to wave energy, might have played an important role in the degradation of the spilled residual fuel oil in the study case.
Acknowledgements
We thank the Marine and Coastal Environment Division, Ministry of the Environment, and the Israel Electric Corporation Ltd. for their generous assistance and Professor Zeev Aizenshtat for his helpful discus-sions. Professor J. R. Maxwell, Dr. K. Grice and Dr. C.A. Lewis are sincerely thanked for their very con-structive reviews of this paper.
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